Creating a model for Garrys mod. How can I copy/paste my model in the edit mode to create a phys model?
one way is (the tutorial one):


select the object (be sure you are in "object mode" and not in "edit mode")
control c to copy
control v to paste
g (move), x (move only on X axis - so you can position better your object), move the mouse, left click when you want to drop the copied object


the other is:


select the object you need to copy (be sure you are in "object mode" and not in "edit mode")
shift d (duplicate)
x (move only on X axis - so you can position better your object)
left click to position the object


otherwise you can do your copy stuff directly inside "scene collection" 
right click, copy, bla bla - see gif -->